The "printing ink" is a cell-seeded alginate hydrogel. The alginate hydrogel is similar to that used for injection molding tissue engineering, but has been modified to be compatible with extrusion through a printing deposition tool. The material is also stiff enough to prevent material sag, hold its shape and be manipulated.

Hod Lipson, director of the Computational Synthesis Laboratory at Cornell University, brought a 3D printer to the conference (annual meeting of the American Association for the Advancement of Science) , to demonstrate how his well-established project, named Fab@Home, is branching out into bioprinting - by creating an ear.

For the demonstration, the real cells that the group would normally use have been replaced with silicone gel in order to bioprint the shape.

Bone repaired with bioprinting (Biofabrication journal/D Cohen) The technique has already been used to print repairs into real animal bones

The team has also published its results from bioprinting repairs in damaged animal bone.

Tissue engineering holds great promise for injury repair and replacement of defective body parts. While a number of techniques exist for creating living biological constructs in vitro, none have been demonstrated for in situ repair. Using novel geometric feedback-based approaches and through development of appropriate printing-material combinations, we demonstrate the in situ repair of both chondral and osteochondral defects that mimic naturally occurring pathologies. A calf femur was mounted in a custom jig and held within a robocasting-based additive manufacturing (AM) system. Two defects were induced: one a cartilage-only representation of a grade IV chondral lesion and the other a two-material bone and cartilage fracture of the femoral condyle. Alginate hydrogel was used for the repair of cartilage; a novel formulation of demineralized bone matrix was used for bone repair. Repair prints for both defects had mean surface errors less than 0.1 mm. For the chondral defect, 42.8 ± 2.6% of the surface points had errors that were within a clinically acceptable error range; however, with 1 mm path planning shift, an estimated ~75% of surface points could likely fall within the benchmark envelope. For the osteochondral defect, 83.6 ± 2.7% of surface points had errors that were within clinically acceptable limits. In addition to implications for minimally invasive AM-based clinical treatments, these proof-of-concept prints are some of the only in situ demonstrations to-date, wherein the substrate geometry was unknown a priori. The work presented herein demonstrates in situ AM, suggests potential biomedical applications and also explores in situ-specific issues, including geometric feedback, material selection and novel path planning techniques.

Early stages but a promising vision

But the method is still in its infancy, and several technical hurdles lie between the groups' current efforts and a future in which injured body parts are repaired digitally on-site or simply printed out fresh.

"Some tissues can be handled more easily than others," Professor Lipson said.

"We and our colleagues have started with cartilage; it's amorphous, it doesn't have a lot of internal structure and vascularisation - that's the entry level point to start with.

"That has been fairly successful in animal models, and that would be the first thing you'll see used in practice. From there we'll climb the complexity of tissue, going to bone, or perhaps liver."

Another concern is that bioprinted tissues aren't easy to connect to the real thing.

"One of the advantages of using the computerised printing is that you can create a tissue construct in a more accurate manner than when you're trying to build something manually," Professor Yoo said.

"But how can we create and connect those tissues produced outside the body? Whatever you put in the body has to be connected with the body's blood vessels, blood supply and oxygen. That's one of the challenges we'll face with larger tissues."